This invention relates to an on-site, continuous method of tracer generation that can be utilized to tag natural gas. Natural gas is composed primarily of methane but contains lesser proportions of many compounds. Notable among those compounds are ethane, propane, and higher hydrocarbons. Although this invention finds application in tagging natural gas feedstock, it can be used to tag many other carbonaceous compounds including pure methane. Feedstock as used in this application encompasses natural gas, pure methane, the components of natural gas such as ethane, or any other carbonaceous substance in either liquid or gaseous form.
Most of the natural gas that is used in North America is produced either in the Gulf Coast region or in Northwestern Canada. Yet, most of the gas is used in the Northeast, the Midwest, and the northwestern United States. Therefore, large pipelines crisscross the country to transport natural gas from the producing areas to areas where the gas is used. Natural gas is frequently a byproduct of oil production. To produce oil, one often must also produce natural gas. Thus natural gas is produced year round in oil producing areas. However, there are also areas, which produce only natural gas, without oil. In those areas it is necessary to produce gas continuously, at a controlled rate, to maximize the productivity of a gas field. Further, if gas or oil is produced too rapidly, it can result in groundwater being drawn into the well and can seriously damage or even destroy a well.
Because gas is produced throughout the year but used primarily during the winter months, it is necessary to store natural gas until the months of peak usage. The most common method of storing natural gas is in underground storage reservoirs. Many of these storage reservoirs are areas where natural gas was produced years before. Because these reservoirs were demonstrated to have contained natural gas for millions of years, they provide a natural storage mechanism. Underground storage fields generally consist of porous rocks that are overlain by non-porous and non-permeable rocks. The porous rocks generally have the pore space filled with water. If one drills through the non-porous overlaying rock, or cap rock, one can pump gas into the pore space of the underlying reservoir unit, displacing the water.
There are over 350 such underground storage fields in North America in which gas is pumped underground during the warmer months of the year, and then withdrawn when additional gas is needed during cold periods. Some of these reservoirs are near the producing areas and others are near the end markets, sometimes in populated areas. Although underground storage reservoirs are designed to contain the gas, leakage of gas from these reservoirs does sometimes occur, resulting in a loss to the owner.
There are many scenarios in which identification of gas that has leaked or has been removed from a storage reservoir is critical. For example, if gas migrates to the surface it can enter shallow groundwater, used for drinking water supplies, and can even come to the surface, enter buildings, and result in explosions. Whenever natural gas is detected in the near-surface environment, over or near a gas storage reservoir, it becomes critical to determine if it is naturally occurring, native gas, or if it is gas leaking from the storage reservoir.
Another setting in which gas identification is critical is when there are producing oil and/or gas wells near gas storage fields. There are numerous situations throughout North America where this is the case. Although a gas company may attempt to define and describe the limits of the underground storage reservoir, the natural variations in the earth structure make it extremely difficult to be precise. Thus when gas is produced from a horizon above or adjacent to a gas storage field, the question frequent arises as to the ownership of that gas. If the gas occurs naturally within the rocks, it is the property of the producer. However, if the gas has migrated from a gas storage field, depending upon local laws, it may remain the property of the gas company. There have been numerous disputes throughout the country over the ownership of natural gas.
Thus, the ability to tag natural gas and the consequent capability of identifying the owner of the gas, is of significant value. To identify the source of natural gas, a tracer (like a fingerprint) may be added to the stored natural gas. By detecting the tracer contained in the gas under investigation, one could trace it back to its source. To qualify, the tracer has to satisfy several criteria: a). it must not normally exist in natural gas; b). it should not segregate from stored natural gas; c). it should not decompose rapidly or react with any other components; d). it should not be absorbed by the aquifer; and e). the detection limit should be low (that is the resolution should be high), so that the amount of added tracer can be low.
Natural gas within distribution pipelines in the country is tagged by adding an odorant. This is generally a sulfur bearing mercaptan. Because these mercaptans do no normally exist within natural gas, the presence of a mercaptan within the gas identifies it as pipeline gas. In gas storage reservoirs, mercaptans cannot be used effectively as tracers because, among other reasons, they are very reactive with the rocks. The gas may contain mercaptans when it is injected into a reservoir, but that mercaptan can quickly disappear and not remain with the gas. There are no existing methods of tagging gas prior to gas storage that are simple enough and inexpensive enough to be used on a routine basis as is done for pipeline gas distribution systems.
Many tracers have been tried, including ethylene, propylene, hydrogen, carbon monoxide, and others. Ethylene (C2H4) is one of the best tracers among all the tested tracers because it satisfies all the requirements of a good tracer. Pure ethylene generated offsite and shipped to the storage field has been used. Since the amount of natural gas to be stored is huge, in the range of billions of cubic feet, the use of pure ethylene is too expensive if it is used on a regular basis. Furthermore, commercially available quantities of ethylene are either too large or too small and are thus not suited to continuous use in tagging natural gas storage fields. This invention produces ethylene and other potential tracers at a low cost and in quantities ideal for tagging natural gas with this tracer.
Although there have been several other tracers developed which can be utilized in gas reservoir studies for various purposes, there are none without serious limitation. For example, U.S. Pat. No. 4,551,154 to Malcosky describes an approach where the chemical sulfur hexafluoride and/or chloropentafluoroethane is injected into gas fields to determine ownership. Field tests have indicated that the two compounds were not fully recovered whereas as tracers such as ethylene, were fully recovered. The two tracers appeared to be less mobile than ethylene. Low permeability structures could restrict the migration of these compounds. Further, this system utilizes very expensive chemicals and specialized analytical equipment. Other authorities have determined that sulfur hexafluoride was not deemed to be a suitable tracer in this application due to its instability and reactivity under long-term field conditions and its differing dispersion behavior relative to methane, while yet other authorities maintain that sulfur hexafluoride may have toxicity problems that may preclude its extensive utilization.